Connecting Rods, Crankshaft and Bearings


Connecting Rods


Connecting rods which link the piston assembly to the crankshaft, are made of forged steel to give them strength. During the power stroke, the force of expanding gases is received by the piston head and transmitted to the piston pin. From the piston pin, this force is transmitted to the connecting rod and finally to the crankshaft. The up-and-down motion of the piston is then transformed into rotary motion of the crankshaft by the connecting rod. During the intake, compression and exhaust strokes, the piston receives the force of the crankshaft from the connecting rod.


The connecting rod consists or three parts: the small end, the shank, and the big end. The small end is connected to the piston pin. If the piston pin is designed to rotate on the small end, a bronze bushing is installed on this small end to reduce friction. The shank is the part between the small end and the big end. In the high performance engine, a hole may be drilled through the connecting rod's center to allow the flow of lubricating oil. The shank may be designed center or off-center to the crankpin. The big end of the connecting rod is usually split and connected to the crankshaft at the crankpin. It is always provided with bearings to minimize friction.


Figure 23     Piston, Connecting Rod and Crankpin Assembly



It is very important that the connecting-rod bearings and crankpin have the correct clearance. The bearing of the connecting rod is made of a steel shell, coated on the working surface with babbitt metal. Babbitt metal is a lead alloy that is very good in reducing friction and thus improve engine performance. However, if the clearance between connecting rod bearings and the crankpin is not correctly or properly provided, the babbitt will easily wear out and engine knocking will result.

Connecting-rod bearings must not rotate inside the big end of the connecting rod because the oil holes in the crankpin will be blocked and engine will overheat.


Figure 24     Connecting Rod Bearing



Connecting rods may develop the following troubles from long use:


  1. Worn out bearings and bushings. In this case, replacement of the worn-out parts are necessary.

  2. The shank, because of the force it receives from the piston and crankshaft, may become bent, twisted or offset. Proper tools can correct these troubles.

Figure 25     Connecting Rod Defects




     Crankshafts are made of forged steel. The crankpins and journals must be hard, the inner portions must be resilient enough to withstand the shock of the forces transmitted to them by the connecting rod during the power strokes. Crankshafts will break if they are hardened all over.


Figure 26     Crankshaft Assembly



     All working units of the engine deliver and receive power from the crankshaft. Thus, it is one of the most important parts of the engine. Because of this, it is very carefully designed to possess enough strength to receive and transmit the power needed to operate the engine. The crankshaft must also be very carefully balanced to minimize engine vibration as much as possible. To achieve good balance, balancers at the crankshaft are built in.


     The crankshaft has a flywheel to keep engine momentum uniform. It is to the flywheel where the clutch is assembled. To the flywheel is also attached the ring gear. The ring gear is the engine part which receives power from the starting motor

when engine is started.


     The crankshaft is assembled to the crankcase by the use of main bearing caps. The main bearing caps have surface similar to connecting-rod bearings and must also be provided with correct clearance for lubrication purposes. Main bearings must not rotate with the crankshaft so as not to block the oil holes that were drilled into the crankshaft.


     The vibration damper is found at the front end of the crankshaft to make engine operation smoother when engine speed

changes. The crankshaft gear drives the timing gears, timing chain or crankshaft gear. The crankshaft pulley drives the cooling fan, the water pump, the generator or alternator of the engine and in some cases the air-conditioning unit of the vehicle.


     Since the crankshaft operates under very severe stresses, crankshaft pins and journals may wear out sooner due to dirty

oil, lack of lubrication or wrong fitting of bearings. Crankshafts may also be out of alignment badly affecting engine performance. Special tools and machines are needed to service and recondition crankshafts properly.


The Valve System


     Internal combustion engines are provided with openings leading to the combustion chamber in the cylinder head. These openings permit the fuel mixture from carburetor to flow into the cylinder during the intake stroke. It also allows burned gases to leave the cylinder during the exhaust stroke. The entering or leaving of the gases in the cylinder is controlled by the valves.


     There are two valves for each cylinder: the intake valve that controls the entrance of fuel and the exhaust valve that provides an outlet for the burned gases. The type of valve used on almost all automotive engines is known as the poppet valve and sometimes referred to as mushroom valve. The two main parts of the valve are the head and the stem.


Figure 27     Engine Valve



Parts of the Valve and Valve Operating Mechanisms


     The valve head or the top part of the valve consists of valve face and margin. The valve face is the part that contacts and firmly seals the valve seat when the valves are close. The valve seats, especially those used by the exhaust valves, are made of special steel to withstand heat coming from the exhaust gases.


Figure 28     Poppet Valve




Figure 29     Valve Seat and Guide



     The valve face and the seat have slanting surfaces that match each other. When both surfaces are in contact with each other they create a good fit and prevent gases to pass out.


     The valve margin is a narrow rim between the valve face and the top of the valve. Below the valve head is the valve stem which is shaped like a round rod with a groove at its lower end. This groove holds the valve locks, when the valves are installed in the engine.


     The valve is held and made to operate straight up and down by the valve guide.


Figure 30     Valve with Guide, Springs and Lock



     The valve springs keep the valves tightly seated to the seats when the valves are closed. The valves are opened and closed by the cams of the camshaft. A valve lifter is placed between the lower end of the valve stem and the camshaft. Valve lifters provide a quick but smooth and quiet operation of the valves.


The Valve Train


     The valve and the mechanisms that operate it is called the valve train. There are four kinds of valve trains and they are usually used to differentiate engine types.


     One of the valve train arrangements is called the L-head type. It consists of the camshaft, a valve lifter, valve spring and lock together. The valves of the L-head engine are located on the cylinder block.


Figure 31     L-Head Engine


     Another valve train arrangement is called the I-head engine and sometimes known as the valve-in-head engine. This means that the valves are located in the cylinder head. In addition to the parts of the valve train of L-head engines, the I-head valve train has rocker arms and push rods between the valve lifters and the valves.


Figure 32     I-Head Engine Valve Train


     Another construction of the valve train is used on F-head engines. The F-head engine has a combination of the valve trains of L-head and I-head engines. This means that some of the valves are found in the cylinder block and the others are in the cylinder head. The exhaust valves are usually in the cylinder block and the intake valves in the cylinder head.


Figure 33     F-Head Valve Train


The fourth type of valve train arrangement is called the T-head. The valves of the T-head engine are arranged in such a way that the exhaust valves are on one side of the cylinder block and the intake valves on the other side.


Figure 34     T-Head Valve Train


Valve Timing


     Valve timing refers to the moment when the intake and exhaust valves open and close. The opening and closing action of the valves must be perfectly controlled in relation to the different strokes and positions of the piston. For instance, the exhaust valve should be "timed" or arranged to open at not exactly the moment when the exhaust stroke begins. The valve should start to open by one-eighth turns of the engine before the exhaust stroke so that the burnt gases can be expelled more completely.


     Also, the intake valves are still open by one-eighth turn of the engine before the intake stroke to allow more time for the fuel mixture to enter the cylinder.


     Valve timing is very important in the performance of engines. Timing of the valves is controlled by the shape of the cams in the camshaft. It is also controlled by the positions of the gears or chains of the camshaft and the crankshaft. Changing the correct position of the gear by only one teeth of the

gear can greatly change the timing. As a result, the valves are not opened or closed at the right moment, thus reducing engine power and performance. For this purpose, the crankshaft and the camshaft gears and their chains are marked. This enables mechanics to assemble them in the correct position and correct valve timing.


     The sizes of the camshaft and crankshaft gears are not the same. Their sizes are so synchronized that the camshaft gear must rotate at half the speed of the crankshaft gear in four-cycle engines. This is done because the four-stroke engine requires the camshaft to close and open each valve once for every two revolutions of the crankshaft.